The present invention relates to a plasma generator using microwave and more particularly to such plasma generator used, for example, to wash or etch surface of a workpiece such as metallic piece, semiconductor or insulator or to coat the surface of such workpiece with thin film.
The plasma generator of RF (radio frequency power source) parallel-plate type as shown in FIG. 6 of the accompanying drawings has been most extensively used to generate processing plasma free from magnetic field.
This plasma generator 10 of well known art comprises a plasma generating chamber 11 containing therein a pair of conductive parallel plates 12a, 12b adapted to be power-supplied from a high-frequency power source 13 so that plasma 14 may be generated between the parallel plates 12a, 12b. Referring to FIG. 6, reference numeral 15 designates a vacuum pump and reference numeral 16 designates a gas charging line.
The plasma generator 20 of RF induction type as shown in FIG. 7 has been also extensively used.
This plasma generator 20 of well known art comprises a plasma generating chamber 21 provided therearound with a high frequency coil 22 adapted to be power-supplied from a high-frequency power source 23 so that plasma 24 may be generated within the plasma generating chamber 21.
Referring to FIG. 7, reference numeral 25 designates an auxiliary field coil, reference numeral 26 designates a vacuum pump and reference numeral 27 designates a gas charging line.
An example of the conventional plasma generators using microwave is the plasma generator of surface wave excitation type as shown in FIG. 8.
This plasma generator 30 of well known art comprises wave guide tubes 31, a plasma generating chamber 32 and a dielectric plates 33 interposed between the wave guide tubes 31 and the plasma generating chamber 32 so as to define dielectric windows. With such construction, the surface wave is generated on the surfaces of the dielectric plates 33 facing the plasma generating chamber 32 as power Po in the form of microwave is supplied from the wave guide tubes 31 and this surface wave ionizes gaseous molecules and thereby generates plasma 34.
Referring to FIG. 8, reference numeral 35 designates a vacuum pump and reference numeral 36 designates a gas charging line.
The plasma generator 10 of FIG. 6 is disadvantageous in that the plasma 14 has a diameter of 0.2 m or less, an electron density of 1xc3x971011 cmxe2x88x923 or less and a plasma density homogeneity of xc2x15% or less. As a result, both the plasma area and the electron density are too low to be used for surface processing of semiconductors and/or spray-coating with a thin film.
The plasma generator 20 of FIG. 7 is more advantageous than the plasma generator 10 so far as the electron density is concerned. However, it is impossible to obtain the plasma of homogeneous high electron density unless complicated adjustment is carried out by application of DC or high frequency auxiliary magnetic field by the auxiliary field coil 25.
The plasma generator 30 of FIG. 8 is necessarily accompanied with increased cost because a thickness of the dielectric plates 33 should be sufficiently increased to protect the plasma generating chamber 32 from ambient pressure if it is desired to enlarge the plasma area. Furthermore, it is difficult for this plasma generator 30 to ensure the desired homogeneity of the plasma density.
In view of the situation as has been described above, it is a principal object of the present invention to provide a plasma generator constructed so that the microwave energy may be used to improve the homogeneity of plasma density as well as the electron density and to ensure plasma having a large area.
The object set forth above is achieved, according to the present invention, by a plasma generator adapted to supply microwave energy into a plasma generating chamber to generate plasma within the plasma generating chamber, said plasma generator comprising a plurality of wave guide tubes arranged in parallel one to another to supply microwave energy, each of the wave guide tubes being formed with a plurality of coupling ports arranged intermittently in an axial direction of the wave guide tube and dimensioned so that coupling coefficient thereof become gradually higher toward a distal end of the wave guide tube, and a plurality of dielectric windows provided through the plasma generating chamber in association with the respective coupling ports of the wave guide tubes.
According to the present invention, the coupling ports are dimensioned so that these ports may have coupling coefficients gradually increasing toward the distal ends of the respective wave guide tubes and thereby microwave energy radiated through the respective coupling ports of the respective wave guide tubes may be uniformized.
Accordingly, by appropriately selecting the intervals of the plural wave guide tubes as well as the intervals of the coupling ports, plasma of high and uniform density can be generated by the microwave energy radiated through the coupling ports and the associated dielectric windows into the plasma generating chamber.
Area of the plasma generated in the manner as has been described above can be enlarged by providing the respective wave guide tubes with a plurality of the coupling ports.
The dielectric windows can be separated one from another in association with the respective coupling ports and therefore the size of the windows can be limited. This feature is advantageous also from the view point of the manufacturing cost.
The wave guide tubes are provided with a plurality of the coupling ports of circular or square shape and diameters of these circular or square ports formed through each of the wave guide tubes are gradually enlarged so as to have the coupling coefficient gradually increasing as closer to the distal end of the wave guide tube the coupling ports are.
The plasma generating chamber is formed at positions corresponding to the respective coupling ports of the respective wave guide tubes with openings communicating with the respective coupling ports so that these openings are blocked off by the dielectric plates and thereby the plasma generating chamber is sealed hermetically.
Furthermore, there are provided a plurality of wave guide tubes, each of these wave guide tubes being formed with a plurality of coupling ports adapted to uniformize the microwave energy radiated through these ports and thereby plasma of a large area is generated with high and uniform density.
The invention allows the dielectric windows to be separated one from another and the dielectric windows as thin as possible can be used. In consequence, the plasma generator using microwave can be easily implemented at a rational manufacturing cost.